Chances are, you're reading this thread because your Steam installation was halfway through updating when suddenly Avast Antivirus pops up, telling you that you have malware!
Don't worry, it's a FALSE POSITIVE.
Your computer is not infected, and this will likely be fixed when Avast releases it's next update.
To fix the problem, you should open up Avast and go to "Real-time shields > File System Shield > Expert Settings > Exclusions". From there, you should add your Steam folder to the exclusions list.
If you continue to have problems, you can either wait until Avast releases it's next update or switch Antivirus temporarily. I hope this helps!